Muscle Wasting in a Hyperthyroid Cat Fed Hill's y/d

I have read your blog posts concening the Hill's y/d diet with much interest (1-5). We have been feeding our 13-year-old female cat the Hill's y/d diet for 6 months now. The high serum thyroid values have come down into the high-normal range and her weight loss has stopped. However, she doesn't look like the same cat as she used to be, and we have noticed her getting smaller and more bony. She eats the y/d, but not with the same gusto as she used to eat when we fed her regular OTC canned foods.

Is this the muscle wasting you talk about in your blogs? Should we change her treatment and put her on medication in order for her to have a more balanced diet?

Although she is not very old, the procedures listed to "cure" hyperthyroidism look like they may be risky. We don't want our cat to undergo anesthesia or have surgery. Is the radioactive treatment dangerous to our cat's health? What does the radioiodine procedure involve? Is it expensive? Does this procedure make it dangerous to handle the cat litter for awhile?

We want my cat to live a long and happy life, but it looks like she won't be able to keep going very long if we keep her on this diet! Thanks for your advice.

My Response:

Muscle wasting and protein needs in older cats
I agree that it is likely that your cat looks "smaller" because of the loss of muscle mass that can occur in  hyperthyroid cats if not fed enough protein (6-9). Like I discuss in my blog posts, Hill's y/d is not a high protein diet, containing less than 28% of calories as protein (1). In contrast, cats in the wild generally eat between 50-60% of their daily calories as protein.

A recent paper proves that cats, as obligate carnivores, require over 5 grams of protein each day to maintain muscle mass (10). Hyperthyroid cats would be expected to need even more protein, since hyperthyroidism is a catabolic state and it's well known that we see muscle breakdown and muscle loss with this disease (2,6,7). Putting that amount of protein into perspective, that means that we should be feed our cats a diet that contains at least 40% of their calories as protein to restore or regain lost muscle mass, an amount much greater than that contained in the y/d diet.

Best treatment for a young hyperthyroid cat without concurrent disease
In my opinion, we should try to remove or destroy the thyroid tumor in any cat that is relatively young and in good health otherwise. At 13-years of age, your cat certainly is not very old (only about 55-60 in human years). By choosing either surgery or radioiodine, we will be able to cure the thyroid disease and do not have to feed an iodine deficient diet (y/d) or use life-long, twice daily medication (9,11).

In most cats, leaving the thyroid tumor allows it to continue to grow larger and larger, and in some cats, the benign tumor can transpose into a malignant carcinoma (12). Radiodine is generally considered to be the treatment of choice. Compared to surgery, no anesthesia is needed, and we don't have to worry about parathyroid damage (the 4 little glands around the thyroid that control calcium) or possible hypocalcemia. For more information about the pros and cons of radioiodine, see my hospital website (www.animalendocrine.com). I also have a FAQ section on the website that should answer all of your questions about this treatment.

If done properly, radioiodine is not at all risky (12). At our facility, most cats only stay with us for 3-5 days. The doses of radioiodine we use are actually quite small in most cats, so their whole body radiation levels are not very high and no side effects are expected.  To make it easier for you to be separated from your cat, we have even installed webcams within each of their condos so you can check-in whenever you wish.

Like you, I too want all cats to live a long and heathy life. In cats with hyperthyroidism, I believe this includes curing the disease so we can focus on proper nutrition (6,7) and environmental enrichment. By not giving life-long medication, this also reduces the cat's level of stress, which we do see in some cats that have to be chronically medicated. And what cat really wants to eat a single type of food every day for the rest of their lives?

References:

1. Peterson ME. Is Hill's y/d a nutritious diet for hyperthyroid cats? Insights into Veterinary Endocrinology. September 28, 2011. 
2. Peterson ME. Optimal protein requirements for older cats and cats with hyperthyroidism. Insights into Veterinary Endocrinology. November 7, 2011. 
3. Peterson ME. Is the protein content of Hill's y/d too low to restore and maintain muscle mass in cats with hyperthyroidism? Insights into Veterinary Endocrinology. November 10, 2011. 
4. Peterson ME. Can increasing the amount of fat or carbohydrate in a cat's diet compensate for low protein intake? Insights into Veterinary Endocrinology. December 22, 2011. 
5. Peterson ME. Is it feasible for older cats to ingest the optimal amounts of protein they need each day? Insights into Veterinary Endocrinology. December 14, 2011. 
6. Peterson ME. Nutritional management of endocrine disease in cats. Proceedings of the Royal Canin Feline Medicine Symposium 2013; 23-28 2013;23-28.
7. Peterson ME. The best diet to feed hyperthyroid cats. Insights into Veterinary Endocrinology. September 13, 2011. 
8. Sparkes AH. Feeding old cats— An update on new nutritional therapies. Topics in Companion Animal Medicine 2011;26:37-42.
9. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders. pp. 571-592, 2012.
10. Laflamme DP, Hannah SS. Discrepancy between use of lean body mass or nitrogen balance to determine protein requirements for adult cats. J Fel Med Surg 2013; in press. 
11. Peterson ME, Broome MR. Radioiodine for hyperthyroidism. In: Bonagura JD, Twedt DC (eds): Current Veterinary Therapy V. Philadelphia, Saunders Elsevier, 2013; in press.
12. Peterson ME, Broome MR. Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. J Vet Intern Med 2012;26:1523.